Monitoring system and method

ABSTRACT

A method and system for monitoring a voltage of a battery cell or a battery stack. A first monitoring unit has a first plurality of battery monitoring nodes, first and second data ports, a first supply port switchably coupled to the first plurality of battery monitoring nodes, a second supply port switchably coupled to the first plurality of battery monitoring nodes, and a third supply port. A controller is connected to the first monitoring unit. Alternatively, a reference voltage may be connected to the controller or it may be connected to the first monitoring unit.

TECHNICAL FIELD

The present invention relates, in general, to batteries and, more particularly, to monitoring a battery stack.

BACKGROUND

Battery stacks are used in many applications including automotive, aerospace, airline, nautical, computer, communications, heavy equipment, remote sensing, etc. The battery stacks are typically arranged in series with one another to provide a power supply of a particular rated voltage to drive an electrical load. Each battery within the battery stack comprises a number of individual battery cells. The lifetime of the battery is strongly dependent on the way in which the battery is charged and discharged and will be reduced by over-charging the cells or over-discharging the cells. In addition, it is desirable to keep all the cells of a battery stack at the same capacity. This corresponds to keeping all the cells at about the same open circuit voltage. Use of the battery and over-discharge of one cell will impact the lifetime of that cell and of the battery. For example, the capacity of a battery cell that is 50 millivolts (mV) different from a battery cell operating at its nominal voltage is 5 percent (%) lower than the capacity of the battery cell operating at its nominal voltage. This difference may be unacceptable in the applications described above. Thus, battery manufacturers are constantly striving to find better and more accurate measurement techniques for measuring the voltage across battery cells. Along with improving measurement techniques, battery manufacturers are searching for ways to balance the cell voltages within a battery stack.

It has been found that one source of error in a battery monitoring and control unit is the voltage reference source of the unit. These units are typically only accurate to ±1% over the temperature range from −40 degrees Celsius (° C.) to 150° C. For a maximum cell voltage of 4.2V this gives a possible range of error of ±42 mV, which is typically too high for many battery applications.

Accordingly, it would be advantageous to have a circuit and a method for monitoring and balancing the voltage of a battery stack and the voltages of the cells within a battery stack. It would of further advantage for the circuit and method to be cost efficient.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from a reading of the following detailed description, taken in conjunction with the accompanying drawing figures, in which like reference characters designate like elements and in which:

FIG. 1 is a schematic diagram of a portion of a battery system in accordance with an embodiment of the present invention;

FIGS. 2A, 2B, and 2C are a schematic diagram of a battery system in accordance with an embodiment of the present invention;

FIG. 3 is a schematic diagram of a portion of the battery system of FIG. 1;

FIG. 4 is a schematic diagram of another portion of the battery system of FIG. 1;

FIGS. 5A, 5B, and 5C are a schematic diagram of a battery system in accordance with another embodiment of the present invention;

FIG. 6 is a schematic diagram of a battery system in accordance with another embodiment of the present invention; and

FIG. 7 is a schematic diagram of a switch suitable for use with battery systems in accordance with the present invention.

DETAILED DESCRIPTION

Generally, the present invention provides a monitoring system and a method for monitoring one or more battery cells or one or more batteries. In accordance with an embodiment of the present invention, a first monitoring unit is used to measure a voltage of a first cell of a first battery and to generate a first measurement result. Measuring the voltage of the first cell of the first battery includes using a first reference voltage. A second monitoring unit is used to measure the first reference voltage and to generate a second measurement result. A first correction factor suitable for use in correcting a set of measurements from at least one of the first and second monitoring units is calculated using the first measurement result and the second measurement result.

In accordance with another embodiment, a first monitoring unit has a first plurality of battery monitoring nodes, first and second data ports, a first supply port switchably coupled to the first plurality of battery monitoring nodes, a second supply port switchably coupled to the first plurality of battery monitoring nodes, and a third supply port. A controller has at least first, second, and third nodes, the first node coupled to the first data port, the second node coupled to the second supply port, and the third node coupled for receiving a reference voltage.

In accordance with yet another embodiment, a monitoring system, comprises a first monitoring unit that has a first plurality of battery monitoring nodes, first and second data ports, a first supply port switchably coupled to the first plurality of battery monitoring nodes, a second supply port switchably coupled to the first plurality of battery monitoring nodes, a third supply port, and a first reference port switchably coupled to the first plurality of battery monitoring nodes and second reference port switchably coupled to the first plurality of battery monitoring nodes. A controller has at least one port coupled to the first data port.

Referring to FIG. 1, a local battery module 22 _(m) comprising a battery unit 20 _(m) coupled to a battery monitor control unit 16 _(m) is illustrated. It should be noted that the subscript “m” represents an integer. Battery monitor control unit 16 _(m) is comprised of a high side interface (IFH) circuit 26 _(m) and a low side interface (IFL) circuit 28 _(m) coupled to a logic circuit 30 _(m). IFH circuit 26 _(m) is referred to as a high side interface circuit because it is connected to a higher voltage potential of battery unit 20 _(m) than IFL circuit 28 _(m). Similarly, IFL circuit 28 _(m) is referred to as a low side interface circuit because it is connected to a lower voltage potential of battery unit 20 _(m) than IFH circuit 26 _(m). IFH circuit 26 _(m) has a terminal coupled to an input/output node 32 _(m) for receiving a source of operating potential V_(CC)E_(m) and a terminal coupled to an input/output node 34 _(m) for transmitting and receiving control signals Tx/Rx1 _(m). IFL circuit 28 _(m) has a terminal coupled to an input/output node 42 _(m) for receiving a source of operating potential V_(CCm) and a terminal coupled to an input/output node 44 _(m) for transmitting and receiving control signals Tx1/Rx2 _(m). Input/output nodes 32 _(m) and 42 _(m) may be referred to as supply nodes and input/output nodes 34 _(m) and 44 _(m) may be referred to as data nodes. Input/output nodes 32 _(m) and 42 _(m) and input/output nodes 34 _(m) and 44 _(m) serve as selectable input ports. Battery monitor control unit 16 _(m) further comprises a selection device 54 _(m) such as, for example, a multiplexer having inputs 54I_(0,m), 54I_(1,m), . . . , 54I_((n−1),m), 54I_(n,m), 54I_((n+1),m), outputs 54O_(1,m) and 54O_(2,m), a positive supply input terminal 54P_(m), and a negative supply input terminal 54M_(m). It should be noted that the subscript “n” represents an integer. Selection device 54 _(m) is not limited to being a multiplexer, but may be a switch, a plurality of switches, or the like.

Supply input terminal 54Pm is connected to an input of IFH circuit 26 _(m) and to output 54O_(2,m) through a switch 62 _(m). IFH circuit 26 _(m) is coupled to supply node 32 _(m) for receiving source of operating potential V_(CC)E_(m), and to supply input terminal 54P_(m), which serves as a local ground. Supply input terminal 32 _(m) is connected to an input of IFH circuit 26 _(m) and to output 54O_(1,m) through a switch 63 _(m). Output 54O_(2,m) is connected to an input of a gain stage 64 _(m) through a switch 66 _(m) and to a source of operating potential such as, for example, ground through a switch 70 _(m), i.e., output 54O_(2,m) is switchably coupled to the input of gain stage 64 _(m) through switch 66 _(m) and to the source of operating potential through switch 70 _(m). Output 54O_(1,m) is connected to another input of gain stage 64 _(m) through a switch 68 _(m), i.e., output 54O_(1,m) is switchably connected to the other input of gain stage 64 _(m). An output of gain stage 64 _(m) is connected to an input of an analog-to-digital converter (ADC)72 _(m). By way of example, gain stage 64 _(m) is an operational amplifier having an inverting input, a non-inverting input, and an output. Thus, output 54O_(2,m) is switchably connected to the inverting input of operational amplifier 64 _(m), output 54O_(1,m) is switchably connected to the non-inverting input of operational amplifier 64 _(m), and the output of operational amplifier 64 _(m) is connected to an input of an analog-to-digital converter (ADC)72 _(m). One or more outputs 72 _(q) of ADC72 _(m) is connected to one or more corresponding inputs of logic circuit 30 _(m). By way of example, ADC72 _(m) is a 10 bit analog-to-digital converter and therefore has ten outputs 72 _(q) coupled to corresponding inputs of logic circuit 30 _(m). ADC72 _(m) generates measurement results that appear at outputs 72 _(q). A control input of ADC72 _(m) is coupled for receiving a source of operating potential such as, for example V_(CCm). A voltage regulator 76 _(m) has an input coupled for receiving a reference potential or voltage reference V_(REFm) from a reference voltage source 75 _(m), an input coupled to supply input terminal 54P_(m) for receiving another source of operating potential V_(PP), and an input coupled for receiving a source of operating potential such as, for example, ground, at input 54M_(m). The output of voltage regulator 76 _(m), i.e., supply VCC_(m), is commonly connected to the reference input of ADC72 _(m) and as a supply of IFL circuit 28 _(m). Source of operating potential V_(CCm), i.e., supply V_(CCm), is derived from reference voltage V_(REFm). It should be noted that reference voltage source 75 _(m) serves as a voltage reference and the output of voltage regulator 76 _(m) serves as supply output VCC_(m) and as a supply and a reference for ADC72 _(m). Supply VCCm also serves as a supply input for battery monitor control unit 16 _(m−1). Preferably, an external capacitor 77 _(m) is coupled between supply VCC_(m) and node 54M_(m) to stabilize supply voltage VCC_(m). A capacitor 78 _(m) has a terminal connected to node 36 _(m) and a terminal connected to node 38 _(m). Nodes 36 _(m) and 38 _(m) are also referred to as stabilization nodes or ports.

In accordance with an embodiment of the present invention, multiplexer 54 _(m) comprises a switch 80 _(0,m) having a terminal that serves as input 54I_(0,m) and a terminal connected to output 54O_(1,m) and a switch 80 _((n+1),m) having a terminal that serves as input 54I_((n+1),m) and a terminal connected to output 54O_(2,m). In addition, multiplexer 54 _(m) includes a switch 80 _(1,m) having a terminal that serves as input 54I_(1,m,) a terminal connected to output 54O_(1,m) and a terminal connected to output 54O_(2,m), a switch 80 _(2,m) having a terminal that serves as input 54I_(2,m), a terminal connected to output 54O_(1,m) and a terminal connected to output 54O_(2,m), a switch 80 _(3,m) having a terminal that serves as input 54I_(3,m), a terminal connected to output 54O_(1,m) and a terminal connected to output 54O_(2,m), . . . , a switch 80 _((n−1),m) having a terminal that serves as input 54I_((n−1),m), a terminal connected to output 54O_(1,m) and a terminal connected to output 54O_(2,m), and a switch 80 _(n,m) having a terminal that serves as input 54I_(n,m), a terminal connected to output 54O_(1,m) and a terminal connected to output 54O_(2,m). By way of example, switches 80 _(0,m) and 80 _((n+1),m) are single pole single throw switches and switches 80 _(1,m), . . . , 80 _(n,m) are single pole double throw switches.

Briefly referring to FIG. 7, an alternative switch network 80A suitable for use in multiplexer 54 _(m) is illustrated. In accordance with embodiments of the present invention, switches 80 _(0,m), 80 _(1,m), 80 _(2,m), . . . , 80 _((n−1),m), 80 _(n,m), 80 _((n−1),m) can be replaced with switching network 80A. Switching network 80A comprises switches 226 and 236 where terminals of switches 226 and 236 are connected to inputs 54I_(0,m), 54I_(1,m), 54I_(2,m), . . . , 54I_((n−1),m), 54I_(n,m), 54I_((n+1),m). The other terminal of switch 226 is connected to terminals of switches 228 and 230. The other terminal of switch 228 is connected to output 54O_(1,m) and the other terminal of switch 230 is connected to terminal 54M_(m). The other terminal of switch 236 is connected to terminals of switches 238 and 240. The other terminal of switch 238 is connected to output 54O_(2,m) and the other terminal of switch 240 is connected to terminal 54M_(m). Switches 226, 228 and 230 are used to connect the top-side of the cell 20 _(n,m) to be measured. The connection for cell 20 _(n,m) from 54I_((n−1),m) to 54O_(1,m) is valid when switches 226 and 228 are closed and switch 230 is open. The connection is broken when switches 226 and 228 are open and the switch 230 is closed to 54M_(m). No cross-coupling through the open-switches 226 or 228 will influence input 54I_((n−1),m) or output 54O_(1,m) because any cross-coupling through the switches 226 or 228 is shorted too 54M_(m). Switches 236, 238 and 240 are used to connect the bottom-side of the cell 20 _(n,m) to be measured. The connection for cell 20 _(n,m) from 54I_(n,m) to 54O_(2,m) is valid when switches 236 and 238 are closed and switch 240 is open. The connection is broken when switches 236 and 238 are open and the switch 240 is closed to 54M_(m). No cross-coupling through the open-switches 236 or 238 will influence input 54I_((n−1),m) or output 54O_(1,m) because any cross-coupling through the switches 226 or 228 is shorted too 54M_(m).

In addition, FIG. 1 illustrates a battery unit 20 _(m) which comprises a series connected plurality of battery cells 20 _(1,m), 20 _(2,m), 20 _(3,m), . . . , 20 _((n−1),m), 20 _(n,m), 20 _((n+1),m). More particularly, a positive terminal of battery cell 20 _(1,m) is coupled to input 54I_(0,m) through a resistor 86 _(0,m), a positive terminal of battery cell 20 _(2,m) and a negative terminal of battery cell 20 _(1,m) are commonly coupled to input 54I_(1,m) through a resistor 86 _(1,m), a positive terminal of battery cell 20 _(3,m) and a negative terminal of battery cell 20 _(2,m) are commonly coupled to input 54I_(2,m) through a resistor 86 _(2,m), a negative terminal of battery cell 20 _(3,m) is coupled to input 54I_(3,m) through a resistor 86 _(3,m), . . . , a positive terminal of battery cell 20 _(n,m) and a negative terminal of battery cell 20 _((n−1),m) are commonly coupled to input 54I_((n−1),m) through a resistor 86 _((n−1)), a positive terminal of battery cell 20 _((n+1),m) and a negative terminal of battery cell 20 _(n,m) are commonly coupled to input 54I_(n,m) through a resistor 86 _(n,m,) and a negative terminal of battery cell 20 _((n+1),m) is coupled to input 54Im,_((n+1)) through a resistor 86 _((n+1),m). A capacitor 88 _(1,m) has a terminal connected to input 54I_(0,m) and a terminal connected to input 54I_(1,m), a capacitor 88 _(2,m) has a terminal connected to input 54I_(1,m) and a terminal connected to input 54I_(2,m), a capacitor 88 _(3,m) has a terminal connected to input 54I_(2,m) and a terminal connected to input 54I_(3,m) . . . , a capacitor 88 _((n−1),m) has a terminal connected to input 54I_((n−1),m), a capacitor 88 _(n,m) has a terminal connected to input 54I_((n−1),m) and a terminal connected to input 54I_(n,m), and a capacitor 88 _((n+1),m) has a terminal connected to input 54I_(n,m) and a terminal connected to input 54I_((n+1),m). A capacitor 90 _(m) has a positive terminal connected to supply input terminal 54P_(m) and a negative terminal connected to supply input terminal 54M_(m). Resistors 86 _(0,m), 86 _(1,m), . . . , 86 _((n−1),m), 86 _(n,m), 86 _((n+1),m) cooperate with corresponding capacitors 88 _(1,m), 88 _(2,m), . . . , 88 _((n−1),m), 88 _(n,m), 88 _((n−1),m) to filter any signal noise that may occur at inputs 54I_(0,m), 54I_(1,m), . . . , 54I_((n−1),m), 54I_(n,m), 54I_((n+1),m). It should be noted that resistors 86 _(0,m), 86 _(1,m), . . . , 86 _((n−1),m), 86 _(n,m), 86 _((n+1),m) and capacitors 88 _(1,m), 88 _(2,m), . . . , 88 _((n−1),m), 88 _(n,m), 88 _((n+1),m) may also be connected in different configurations as long as they filter the signal noise and protect inputs 54I_(0,m), 54I_(1,m), . . . , 54I_((n−1),m), 54I_(n,m), 54I_((n+1),m) against voltage transients that could damage input circuitry.

FIGS. 2A, 2B, and 2C are a schematic diagram of a battery system 10 having battery terminals 12 and 14. For the sake of clarity, battery system 10 has been drawn on three separate sheets where reference characters “A,” “B,” and “C” have been appended to reference character “2” to distinguish the sheets. It should be noted that FIGS. 2A, 2B, and 2C are intended to be viewed together. Battery system 10 is comprised of a plurality of local battery modules 22 _(m) in which battery monitor control units 16 _(m) are coupled in a daisy-chain configuration to a controller 100. It should be noted that the daisy-chain configuration is realized by connecting a data or data input/output node 34 _(m) to a succeeding data or data input/output node 44 _(m). The daisy-chain connection will be further described below. What is shown in FIGS. 2A, 2B, and 2C are battery monitor control units 16 ₁, 16 ₂, . . . , 16 _(m) connected to battery units 20 ₁, 20 ₂, . . . , 20 _(m), respectively. As previously discussed, battery monitor control units 16 _(m) are coupled to corresponding battery units 20 _(m) to form local battery control modules 22 _(m). Thus, battery monitor control unit 16 ₁ is coupled to battery unit 20 ₁ to form a local battery module 22 ₁, battery monitor control unit 16 ₂ is coupled to battery unit 20 ₂ to form a local battery module 22 ₂, . . . , and battery monitor control unit 16 _(m) is coupled to battery unit 20 _(m) to form a local battery module 22 _(m). Battery monitor control units 16 ₁, 16 ₂, . . . , 16 _(m) and corresponding battery units 20 ₁, 20 ₂, . . . , 20 _(m) are not limited to being directly adjacent to each other. Accordingly, battery monitor control units 16 ₁, 16 ₂, . . . , 16 _(m) may be on one or more Printed Circuit Boards (PCBs), while battery units 20 ₁, 20 ₂, . . . , 20 _(m) may be close to the PCB containing the corresponding battery monitor control units 16 ₁, 16 ₂, . . . , 16 _(m). For example, battery monitor control unit 16 ₁ may be on one PCB with battery unit 20 ₁ near battery monitor unit 16 ₁ whereas battery control unit 16 _(m) may be on a different PCB with battery unit 20 _(m) near battery monitor unit 16 _(m). It should be noted that FIGS. 2A, 2B, and 2C illustrate a plurality of battery monitor control units 16 _(m) coupled together and a plurality of battery units 20 _(m) coupled to corresponding battery monitor control units 16 _(m) in a daisy-chain configuration, whereas FIG. 1 illustrates a single battery monitor control unit 16 _(m) to which a single battery unit 20 _(m) is connected. It should be noted that subcripts m and n have been included in FIGS. 1 and 2 to distinguish the circuit elements of battery monitor control units 16 _(m) from each other and to distinguish the circuit elements of battery units 20 _(m) from each other.

Still referring to FIG. 2, a main controller 100 is connected to battery monitor control unit 16 ₁. Main controller 100 has an ADC102 coupled to V_(CC) input 42 ₁ through a gain stage 103 and a data node 104 connected to data node 44 ₁ of battery monitor control unit 16 ₁. An input 106 of main controller 100 is connected to battery terminal 14 of battery unit 20 ₁. Terminal 14 is coupled for receiving a source of operating potential such as, for example, ground, which serves as a local ground. An external reference voltage 108 for providing a reference voltage V_(REF) is connected to main controller 100.

In operation, battery monitor control unit 16 ₁ and controller 100 communicate with each other through data nodes 44 ₁ and 104. Because battery monitor control unit 16 ₁ and controller 100 are connected in a daisy-chain configuration that is bidirectional, controller 100 can send data to or receive data from battery monitor control units 16 ₁, . . . , 16 _(m). Controller 100 is capable of sending data to or receiving data from selected battery monitor control units 16 ₁, . . . , 16 _(m). In accordance with an embodiment, controller 100 assigns addresses for the battery monitor control units 16 ₁, . . . , 16 _(m) at initialization. Accordingly, at start-up, addresses are not assigned to battery monitor control units 16 ₁, . . . , 16 _(m). Controller 100 communicates with battery monitor control unit 16 ₁, i.e., the battery monitor control unit to which it is directly connected, and assigns it an address. Thus, battery monitor control unit 16 ₁ serves as a slave. Once battery monitor control unit 16 ₁ has an address, controller 100 allows it to send a command to the next battery monitor control unit and controller 100 assigns an address to this battery monitor control unit. Once this second battery monitor control unit has received an address, the communication to the next battery monitor control unit is enabled. Controller 100 continues this process until all the battery monitor control units are enabled. After controller 100 assigns the addresses, it can communicate with each of battery monitor control units 16 ₁, . . . , 16 _(m). The daisy chain connection through data nodes 44 ₁ and 104 can be a single connection for single ended communication, two connections for differential communication, or a serial peripheral interface (SPI), or the like. Differential communication permits communication at higher baud rates with a more acceptable electromagnetic emission than can be achieved with single ended communication.

Controller 100 instructs battery monitor control unit 16 ₁ to measure the voltages of cells 20 _(1,1), . . . , 20 _((n−1),1), 20 _(n,1), 20 _((n+1),1). In response to the instructions from controller 100, logic circuit 30 ₁ actuates switches 80 _(0,1), . . . 80 _((n−1),1), 80 _(n,1), 20 _((n+1),1) of multiplexer 54 ₁ so that the voltage across the desired cell can be measured, i.e., switches 80 _(0,1), . . . , 80 _((n−1),1), 80 _(n,1), 80 _((n+1),1) are set to select a pair of input nodes 54I_(0,1), 54I_(1,1), . . . , 54I_((n−1),1), 54I_(n,1), 54I_((n+1),1) which are connected to the desired cell to be measured. For example, to measure cell 20 _(2,2) of battery unit 20 ₂, ports 54I_(1,1), and 54I_(2,1) are selected as inputs. Capacitor 78 ₂ charges to the potential of the cell under test. Once capacitor 78 ₂ is charged, multiplexer 54 ₂ is opened and capacitor 78 ₂ floats. Logic circuit 30 ₂ closes switches 68 ₂, 66 ₂, and 70 ₂ thereby applying the voltage that is across capacitor 78 ₂ as an input to gain stage 64 ₂. The output of gain stage 64 ₂ is applied to ADC72 ₂, which uses voltage VCC₂ as a reference voltage. ADC72 ₂ generates an output signal that is a digital value of the voltage for cell 20 _(2,2) and serves as the measurement data. Typically, battery monitor control units 16 ₁, . . . , 16 _(m) take measurements in a cyclic fashion and store the data in corresponding storage elements until controller 100 requests the information. The data is stored in storage elements (not shown) that form portions of logic circuits 30 ₁, . . . , 30 _(m). Alternatively, the measurement data can be transferred directly to controller 100. In a next cycle, capacitor 78 ₂ is charged to the voltage of another cell under test, e.g., one of cells 20 _(1,2), 20 _(3,2), . . . , 20 _((n+1),2). It should be noted that the next measurement can be performed without discharging capacitor 78 ₂ and that the desirable charging of capacitor 78 ₂ can be achieved by adding extra diagnostics to ensure that none of pins 54I_(2,n) is open or floating. Typically, controller 100 causes the cells of a battery unit 20 _(m) to be measured in a sequence such that the first cell measured is between pair of nodes 54I_(0,2) and 54I_(1,2) and the last cell measured is between nodes 54I_(n,2) and 54I_((n+1),2).

Switches 62 ₂ and 63 ₂ are included to measure voltage V_(CC)E₂. More generally, switches 62 _(n) and 63 _(n) are included to measure corresponding voltages V_(CC)E_(m).

In accordance with embodiments of the present invention, a calibration or correction factor CORR₁₀₀ for battery monitor control unit 16 ₁ can be determined using the circuit elements shown in FIG. 3. ADC102 is used to measure voltage V_(CC1) and the resulting correction factor is used to correct the error of battery monitor control unit 16 ₁. What is shown in FIG. 3 is ADC102 coupled for receiving an input signal from gain stage 103 and a reference voltage V_(REF108) from a precision external reference voltage source 108. Gain stage 103 receives a voltage V_(CC1) from battery monitor control unit 16 ₁ and ADC102 outputs digital values ADC₁₀₂ for main controller 100. Voltage V_(CC1) output by voltage regulator 76 _(m) is given by Equation 1 (EQT. 1) as:

V _(CC1) =K ₁₀₃*(ADC _(102(VREF108)) /FS ₁₀₀)*V _(REF108)   EQT. 1

where:

K₁₀₃ is a gain factor for gain stage 103 and ADC₁₀₂;

ADC_(102(VREF108)) is the digital value for the voltage from voltage regulator 76 ₁ that is output by ADC102;

FS₁₀₀ is a full scale factor for ADC102; and

V_(VREF108) is a reference voltage derived from precision reference voltage source 108.

Setting voltage V_(CC1) equal to V_(REF108) in EQT. 1 and solving for gain factor K₁₀₃ yields EQT. 2, which is used in the calibration for ADC102.

K ₁₀₃ =FS ₁₀₀ /ADC _(102(VREF108)).   EQT. 2

Substituting gain factor K₁₀₃ of EQT. 2 into EQT 1 yields EQT. 3:

V _(CC1)=(ADC _(102(VCC1)) /ADC _(102(VREF108)))* V _((VREF108)).   EQT. 3

The correction factor CORR₁₀₀ is defined as the ratio of the voltage at the output of voltage regulator 76 ₁, i.e., V_(CC1), to the voltage at the output of reference voltage source 108, i.e., V_(REF108). Rearranging EQT. 3 yields EQT. 4, i.e., an equation defining correction factor CORR₁₀₀:

CORR ₁₀₀ =V _(CC1) /V _(REF108) =ADC ₁₀₂(VCC1 )/ADC _(102(VREF108)).   EQT. 4

Correction factor CORR₁₀₀ is used to correct the voltages of battery cells 20 _(1,1), . . . , 20 _(n,1) of battery monitor control unit 16 ₁.

Using the schematic shown in FIG. 4, the cell voltage V_(20n,1) of battery cell 20 _(n,1) of local battery module 22 ₁ is given by EQT. 5 as:

V _(20n,1) =K ₁*(ADC72_(1(20n,1)) /FS ₁)*V _(CC1)   EQT. 5

where:

K₁ is a gain factor for gain stage 64 ₁ and ADC72 ₁;

ADC72 _(1(20n,1)) is the digital value for battery cell 20 _(n,1) that is output by ADC72 ₁;

FS₁ is a full scale factor for ADC72 ₁; and

V_(CC1) is a source of operating potential derived from local reference V_(REF1).

In the calibration process for ADC72 ₁, setting voltage V_(20n,1) equal to voltage V_(CC1) in EQT. 5 and solving for gain factor K₁ yields EQT. 6:

K ₁ =FS ₁ /ADC72_(1(20n,1))   EQT. 6

Substituting gain factor K₁ of EQT. 6 into EQT. 5 yields EQT. 7:

V _(20n,1)=(ADC72_(1(20n,1))/ADC72_(1(VCC1)) *V _(CC1)   EQT. 7

Substituting correction factor CORR₁₀₀ into EQT. 7 yields EQT. 8, which gives the corrected cell voltage VCORR_(20n,1):

VCORR _(20n,1)=(ADC72_(1(20n,1)) /ADC72_(1(VCC1)))*CORR ₁₀₀ *V _((VREF108)).   EQT. 8

Correction factors can be determined for each battery monitor control unit 16 _(m) using the circuit elements shown in FIG. 4 and equations similar to equations 1-8. For example, a correction factor CORR₁ may determined as follows:

V _(CC2)=(ADC72_(1(VCC2)) /ADC72_(1(VCC1)))*V _(CC1)

where:

ADC72 _(1(VCC1)) is the digital value for the voltage from voltage regulator 76 ₁ that is output by ADC72 ₁;

ADC72 _(1(VCC2)) is the digital value for the voltage from voltage regulator 76 ₂ that is output by ADC72 ₁.

The correction factor CORR₁ for battery monitor control unit 16 ₂ is given by EQT. 9:

CORR ₁ =V _(CC2) /V _(CC1)=(ADC72_(1(VCC2)))/ADC72_(1(VCC1))).   EQT. 9

The cell voltage V_(20n,2) of battery cell 20 _(n,2) of local battery module 22 ₂ is given by EQT. 10 as:

V _(20n,2)=(ADC72_(2(20n,2)) /ADC72_(2(VCC2)))*V _(CC2)   EQT. 10

where:

ADC72 _(2(20n,2)) is the digital value for battery cell 20 _(n,2) that is output by ADC72 ₂; and

V_(CC2) is a source of operating potential derived from local reference V_(REF2).

Substituting correction factor CORR₁ into EQT. 10 yields EQT. 11, which gives the corrected cell voltage VCORR_(20n,2):

VCORR _(20n,2)=(ADC72_(2(20n,2)) /ADC72_(2(VCC2)))*CORR ₁ *CORR ₁₀₀ *V _((VREF108))   EQT. 11

By combining all the correction factors, the measurements of all the cells are correlated with the same precision reference, i.e., voltage reference 108. Thus, corrected cell voltages VCORR_(20n,m) are given by:

VCORR _(20n,m)=(ADC72_(m(20n,m))/(ADC72_(m(VCCm)))*CORR _((m−1)) * . . . *CORR ₂ *CORR ₁ * CORR ₁₀₀ * V _((VREF108)).   EQT 12

Controller 100 applies the corresponding correction factor to the output signals of corresponding analog-to-digital converters ADC72 ₁, ADC72 ₂, . . . , ADC72 _(m).

In a calibration step, logic circuits 30 _(m) generate control logic signals which cause battery monitor control units 16 _(m) to perform a series of measurements that include measuring zero voltages across sampling capacitors 78 _(m) and measuring local reference voltages V_(CCm) across sampling capacitors 78 _(m). The zero voltages across sampling capacitors 78 _(m) are used to correct voltage offset that may appear in battery control units 16 _(m) and the local reference voltage measurement, i.e., the measurement of voltages V_(CCm), is used to calibrate the gain of the measurement channel. The zero voltage across capacitors 78 _(m) is measured by setting the voltages at nodes 36 _(m) and 38 _(m) equal to each other and the voltage at node 38 _(m) substantially equal to a ground potential. The local voltage reference measurement is achieved by setting the voltage at node 36 _(m) substantially equal to reference voltage V_(CCm) and the voltage at node 38 _(m) substantially equal to a ground potential. The voltages between these two points can be corrected using interpolation or through a local correction factor of ADC72 _(m).

FIGS. 5A, 5B, and 5C are a schematic diagram of a battery system 150 in accordance with another embodiment of the present invention. For the sake of clarity, battery system 10 has been drawn on three separate sheets where reference characters “A,” “B, ” and “C” have been appended to reference character “5” to distinguish the sheets. It should be noted that FIGS. 5A, 5B, and 5C are intended to be viewed together. Like battery system 10, battery system 150 is comprised of a plurality of local battery modules in which battery monitor control units 16A_(m) are coupled in a daisy-chain configuration to a controller. Battery monitor control units 16A_(m) are similar to battery monitor control units 16 _(m), except that they have additional switches and input/output nodes. Thus, battery system 150 differs from battery system 10 in that each battery monitor control unit includes a switch 154 _(m) having a terminal connected to node 54O_(2,m) and a terminal connected to a node 160 _(m) and a switch 156 _(m) having a terminal connected to node 54O_(1,m) and a terminal connected to a node 158 _(m). In addition, a precision external reference voltage 164 is connected to at least one set of nodes 158 _(m) and 160 _(m) rather than to a controller 152. By way of example, precision external reference voltage 164 is connected to nodes 158 ₂ and 160 ₂. The battery monitor control unit 16A₂ to which precision external reference voltage 164 is connected is not a limitation of the present invention. The precision external reference voltage can be connected to battery monitor control unit 16A₁, 16A₂, 16A₃, or 16A_(m). Controller 152 is connected to nodes 44 _(1A) and 44 _(1B) through signal couplers 166 and 168, respectively.

Correction factors for battery system 150 can be determined in a fashion similar to that for determining the correction factors for battery system 10. For example, the correction factors when integer “m” is three are given in EQTS. 13-15 as:

CORR ₁ =V _(CC2) /V _(CC1)=(ADC72_(1(VCC2)))/ADC72_(1(VCC1)));   EQT. 13

CORR ₂ =V _(CC3) /V _(CC2)=(ADC72_(2(VCC3)))/ADC72_(2(VCC2)));   EQT. 14

CORR _(VREF) =V _(CC2) /V _(REF)=(ADC72_(2(VCC2)))/ADC72_(2(VREF))); and   EQT. 15

CORR _(m−1) =V _(CCm) /V _(CC(m−1)) =ADC72_(m−1(VCCm)))/ADC72_(m−1(VCC(m−1))))   EQT. 16

The corrected ADC voltage for ADC72 ₂ can be calculated in the same way as for battery system 10: The uncorrected value is given by EQT. 17 as:

V _(20n,2)=(ADC72_(2(20n,2)) /ADC72_(2(VCC2)))*V _(CC2).   EQT. 17

Applying the correction factor gives EQT. 18:

VCORR _(20n,2)=(ADC72_(2(20n,2)) /ADC72_(2(VCC2)))*CORR _(VREF) *V _(REF).   EQT. 18

The corrected value for ADC72 ₁ can be obtained in the same way and is given by EQT. 19 and EQT. 20 as:

V _(20n,1)=(ADC72_(1(20n,1)) /ADC72_(1(VCC1)))*V _(CC1)   EQT. 19

VCORR _(20n,1)=(ADC72_(1(20n,1)) /ADC72_(1(VCC1)))*1/CORR ₁ *CORR _(VREF) *V _((VREF))   EQT. 20

For battery monitor control units higher than battery monitor control unit 16 ₂, the corrected values are given by EQT. 21 and EQT. 22 as:

V _(20n,m)=(ADC72_(m(20n,m)) /ADC72_(m(VCCm)))*V _(CCm)   EQT. 21

VCORR _(20n,m)=(ADC72_(m(20n,m)) /ADC72_(m(VCCm)))*CORR _((m−1)) * . . . *CORR ₂ *CORR _(VREF) * V _((VREF)).   EQT. 22

Thus, using the different correction factors , all the measured cell voltages can be corrected towards the precision of reference voltage 164.

The number of reference voltages associated with battery monitor control units 16A₁, . . . , 16A_(m) is not a limitation of the present invention. FIG. 6 illustrates a battery system 200 in accordance with another embodiment of the present invention. What is shown in FIG. 6 is a battery system 200 that is similar to battery system 150 except that battery system 200 includes a precision external reference voltage 165 ₁ connected to nodes 158 ₂ and 160 ₂ and a precision external reference voltage 165 ₂ connected to nodes 158 _(m) and 160 _(m). Connecting an extra external reference voltage to more than one battery monitor control unit 16A_(m), i.e., connecting extra external reference voltage 165 ₁ to battery monitor control unit 16A₂ and connecting extra external reference voltage 165 ₂ to battery monitor control unit 16A_(m), improves the overall accuracy and provides redundancy within the battery system. It should be noted that precision external reference voltages 165 ₁ and 165 ₂ may be at the same voltage level or at different voltage levels.

By now it should be appreciated that a method and structure for monitoring one or more batteries have been provided. In accordance with embodiments of the present invention, an internal reference voltage is input to an internal voltage regulator which generates a low voltage local supply voltage (V_(CCm)) that is stabilized by an external capacitor. The local supply voltage is input to another battery monitor control unit at input 32 _(m) as serves as a voltage V_(CC)E_(m). The measurement of the voltage at input 32 _(m), i.e., voltage V_(CC)E_(m) provides an indication of the voltage of the reference voltage used to generate the local supply voltage V_(VCCm). which allows for the calculation of a correction factor for each battery monitor control unit. Connecting battery monitor control units 16 _(m) in a daisy-chain configuration, i.e., the daisy-chain connection of voltages V_(CCm) and V_(CC)E_(m) enables the calculation of a correction factor for each battery monitor control unit 16 _(m) within the daisy-chain. Using a precision voltage reference allows improving the absolute accuracy of the voltages within the daisy-chain. An accuracy of ±0.2% can be achieved. Because the internal reference voltage is used as the reference for ADC's 72 _(m), the accuracy of all the measurements carried out by ADC's 72 _(m) are directly proportional to supply voltage V_(CC). In addition, ADC's 72 _(m) measure voltages V_(CC)E_(m), which allows for correcting of all the ADC values including that of the controller.

Although specific embodiments have been disclosed herein, it is not intended that the invention be limited to the disclosed embodiments. Those skilled in the art will recognize that modifications and variations can be made without departing from the spirit of the invention. It is intended that the invention encompass all such modifications and variations as fall within the scope of the appended claims. 

1. A method for monitoring one or more batteries, comprising: measuring a voltage of a first cell of a first battery using a first monitoring unit to generate a first measurement result, wherein measuring the voltage of the first cell of the first battery includes using a first reference voltage; measuring the first reference voltage using a second monitoring unit to generate a second measurement result; and using the first measurement result and the second measurement result to calculate a first correction factor suitable for use in correcting a set of measurements from at least one of the first and second monitoring units.
 2. The method of claim 1, further including: measuring a voltage of a second cell of the first battery using the first monitoring unit, wherein measuring the voltage of the second cell includes using the first reference voltage; and using the first correction factor to correct the measurement of the voltage of the second cell.
 3. The method of claim 1, further including measuring voltages of a plurality of additional cells of the first battery using the first monitoring unit, wherein measuring the voltages of the plurality of additional cells includes using the first reference voltage; and using the first correction factor to correct the measurements of the voltages of the plurality of cells.
 4. The method of claim 1, further including: measuring a voltage of a first cell of a second battery using the second monitoring unit to generate a third measurement result, wherein measuring the voltage of the first cell of the second battery includes using a second reference voltage; measuring the second reference voltage using a third monitoring unit to generate a fourth measurement result; and using the third measurement result and the fourth measurement result to calculate a second correction factor suitable for use in correcting a set of measurements from at least one of the first and second monitoring units.
 5. The method of claim 1, further including using the first and second correction factors to correct the set of measurements from at least one of the first, second, and third monitoring units.
 6. The method of claim 1, further including configuring at least one of the first and second monitoring units to measure a voltage across the first cell or a group of cells.
 7. The method of claim 6, further including providing a selectable input port to at least one of the first and second monitoring units, the selectable input port for coupling to a third monitoring unit, the third monitoring unit adjacent to at least one of the first and second monitoring units.
 8. The method of claim 6, further including providing a first additional selectable input port for coupling to an adjacent monitoring unit and that is capable of measuring a voltage that is related to the reference voltage of the adjacent monitoring unit and providing a second additional selectable input port for measuring a voltage of an external reference voltage.
 9. The method of claim 1, further including using a controller to calculate the first and second correction factors.
 10. The method of claim 1, further including: coupling a controller to at least one of the first and second monitoring units; and coupling the first reference voltage to the controller.
 11. The method of claim 1, further including: coupling a controller to at least one of the first and second monitoring units; and coupling the first reference voltage to an additional selectable port to measure a voltage of the first external reference voltage of at least one of the first and second monitoring units.
 12. The method of claim 1, further including: coupling a controller to at least one of the first and second monitoring units; coupling the first reference voltage to the first monitoring unit; and coupling the second reference voltage to the second monitoring unit.
 13. The method of claim 1, further including: measuring first voltage at a first node in the first monitoring unit; measuring a second voltage at a second node in a second monitoring unit; using the first voltage at the first node and the second voltage at the second node to calculate a second correction factor suitable for use in correcting a set of measurements from at least one of the first and second monitoring units.
 14. A monitoring system, comprising: a first monitoring unit having a first plurality of battery monitoring nodes, first and second data ports, a first supply port switchably coupled to the first plurality of battery monitoring nodes, a second supply port switchably coupled to the first plurality of battery monitoring nodes, and a third supply port; and a controller having at least first, second, and third nodes, the first node coupled to the first data port, the second node coupled to the second supply port, and the third node coupled for receiving a reference voltage.
 15. The monitoring system of claim 14, further including a second monitoring unit having a second plurality of battery monitoring nodes, first and second data ports, a first supply port switchably coupled to the second plurality of battery monitoring nodes, a second supply port switchably coupled to the second plurality of battery monitoring nodes, and a third supply port, the first data port of the second monitoring unit coupled to the second data port of the first monitoring unit, and the third supply port of the second monitoring unit coupled to first supply port of the first monitoring unit.
 16. The monitoring system of claim 15, wherein the first monitoring unit further includes: a gain stage having first and second inputs and an output, the first input switchably coupled to a first portion of the first plurality of battery monitoring nodes and the second input switchably coupled to the second plurality of battery monitoring nodes: a converter circuit having first input coupled to the output of the gain stage, a second input coupled to the third supply port of the first monitoring unit, and an output; and a logic circuit having an input node coupled to the output of the converter circuit, a first input/output node coupled to the first data port, and a second input/output node coupled to the second data port.
 17. A monitoring system, comprising: a first monitoring unit having a first plurality of battery monitoring nodes, first and second data ports, a first supply port switchably coupled to the first plurality of battery monitoring nodes, a second supply port switchably coupled to the first plurality of battery monitoring nodes, a third supply port, and a first reference port switchably coupled to the first plurality of battery monitoring nodes and second reference port switchably coupled to the first plurality of battery monitoring nodes; and a controller having at least one port, the at least one port coupled to the first data port.
 18. The monitoring system of claim 17, further including a first reference voltage source having first and second terminals, the first terminal coupled to the first reference port and the second terminal coupled to the second reference port.
 19. The monitoring system of claim 18, further including: a gain stage having first and second inputs and an output, the first input switchably coupled to a first portion of the first plurality of battery monitoring nodes and the second input switchably coupled to the second plurality of battery monitoring nodes: a converter circuit having first input coupled to the output of the gain stage, a second input coupled to the third supply port of the first monitoring unit, and an output; a logic circuit having an input node coupled to the output of the converter circuit, a first input/output node coupled to the first data port, and a second input/output node coupled to the second data port; a first switch having first and second terminals, the first terminal coupled to the first plurality of battery monitor nodes and the second terminal coupled to the first reference port; and a second switch having first and second terminals, the first terminal coupled to the first plurality of battery monitor nodes and the second terminal coupled to the second reference port.
 20. The monitoring system of claim 18, further including: a second monitoring unit having a second plurality of battery monitoring nodes, third and fourth data ports, a fourth supply port switchably coupled to the second plurality of battery monitoring nodes, a fifth supply port switchably coupled to the second plurality of battery monitoring nodes, a sixth supply port, and a third reference port switchably coupled to the second plurality of battery monitoring nodes and a fourth reference port switchably coupled to the second plurality of battery monitoring nodes; and a second reference voltage source having first and second terminals, the first terminal coupled to the third reference port of the second monitoring unit and the second terminal coupled to the fourth reference port of the second monitoring unit. 